This invention relates to a method of curtain coating for coating a variety of coating liquid on a running web, particularly a method for providing a high speed stable coating to a spliced portion of the web. The method is suitable for manufacturing photographic film, photographic paper, pressure sensitive paper, thermal recording paper, ink jet paper and the like.
Curtain coating is performed by forming a free falling liquid curtain which impinges on a running web. The curtain coating is capable of making higher speed coating of simultaneous multiple layer than slide bead coating or slot (extrusion) coating because free-falling and impinging curtain has more ability of removing an entrained air in the boundary layer on the running web than slide bead coating or slot coating.
Many proposals for resolving problems of such air entrainment and liquid puddle formed at the impingement point of the curtain on the side of the approaching web have been presented. For example, Japanese Laid-open Patent Publication 146172/91 (tokkai-hei 3-146172) shows curtain coating for kind-of rough surface web using a lowermost layer having a viscosity more than 90 mPas at low shear rate and relatively lower viscosity at high shear rate in addition to keeping an average viscosity of all the layers more than 80 mPas. Japanese Laid-open Patent Publication 143569/91 (tokkai-hei 3-143569) shows that curtain coating for smooth surface web using a coating liquid having a viscosity between 50 and 100 mPas at low shear rate. PCT National Publication No. 503752/94 (tokuhyou-hei 6-503752) shows curtain coating using a lowermost layer having a viscosity more than 20 mPas at shear rate less than 500 Sxe2x88x921 and less than 10 mPas at shear rate more than 106 Sxe2x88x921. There are many other proposals also focusing on optimizing a viscosity of lowermost layer. Other methods, for example, in Japanese patent 2835659, suggest heating or electrically charging the web to improve high speed coating stability.
Proposals mentioned above are useful to carry out high speed coating but still not sufficient to provide stable coating on a spliced portion of continuously web running at the speed around or more than 300 m/min. Instability or turbulence of coating curtain is caused by passing a spliced portion where a trailing end of old web and a leading end of new web are butted and spliced with splicing (adhesive) tape, more precisely by passing step formed by a trailing edge of the splicing tape and the web surface, which allows air entrainment, puddling and/or slipping of coating liquid and result in forming non-uniform coating thickness portion on the web. Thicker portion can not be dried completely by passing drying zone section, which contaminates web-transporting rollers and causes adhesion between the webs after being wound. Japanese Laid-open Patent Publications 104061/93 (tokkai-hei 5-104061) and 137672/98 (tokkai-hei 10-137672) show charging the spliced portion or electrifying the spliced portion with higher voltage charge than other portion. Those methods are still not perfect to avoid instability caused by the spliced portion.
An object of the invention is to provide method of high speed coating with keeping high stability in spite of spliced portion passing.
It is known that curtain coating is capable of applying more viscous liquid to a web than slide bead coating and viscosity able to be applied by curtain coating is normally between 1 and 200 mPas. We found, however, using coating liquid having viscosity more than 300 mPas, which had been unexpected value, to one or more layer of multiple layers improves amazingly curtain coating stability against disturbance such as passing spliced portion of the web. In addition to that, the one or more layers for which coating liquid having viscosity more than 300 mPas is to be used is selected from intermediate layer, although most of conventional studies to improve coating stability had been focused on optimization of liquid property of uppermost and/or lowermost layer, and the property of intermediate layer had been thought insignificant. It was also found that curtain coating stability is further improved by combining the fact mentioned above with other technologies such as electrifying web, heating web upstream of coating point and/or keeping web tension higher than some specified value. The invention is briefly explained below. Here, the term xe2x80x9cviscosityxe2x80x9d is defined as that measured at shear rate of 10 Sxe2x88x921 unless otherwise defined.
Main part of the invention is as follows.
In method of curtain coating for applying simultaneously multiple layers to a running web, the multiple layers includes a lowermost layer, a uppermost layer and an intermediate layer which includes at least one layer made from coating liquid having viscosity more than 300 mPa. In method of curtain coating for applying simultaneously multiple layers to a running web, the multiple layers includes a lowermost layer, a uppermost layer and an intermediate layer which includes at least one layer made from coating liquid having viscosity more than 300 mPa, and a total flow rate of coating liquid having viscosity more than 300 mPas for the intermediate layer is less than 25% of a total flow rate of coating liquid for all the multiple layers.
Other factors usable to constitute the invention are below. The web includes a spliced portion where an old web and a new web are butted and spliced with an splicing tape, and each surface potential of the splicing tape and the web at least near the spliced portion is between 0.5 and 2.5 kV. The total flow rate of coating liquid having viscosity more than 300 mPas for intermediate layer is preferably between 5 and 15% of the total flow rate of coating liquid for all the multiple layers. The coating liquid having viscosity more than 300 mPas for intermediate layer preferably has a viscosity between 80 and 300 mPas at shear rate 1000 Sxe2x88x921. The coating liquid having viscosity more than 300 mPas for intermediate layer preferably has a viscosity between 300 and 500 mPas and a viscosity of each coating liquid for other layers of the multiple layers is preferably less than 200 mPas. The web is preferably heated so that the web can keep its surface temperature between 30 and 50xc2x0 C. at the point where a free falling curtain impinges the web. A tension applied to the web around a backing roller is recommended to be more than 8xc3x97105 N/m2.